Reflecting the recent enhancement of global environmental control, various automobile manufacturers are affirmatively conducting development of reducing pollution by exhaust gas as well as reducing fuel consumption. Transition in the conventional engine from the mechanical control mechanism to the electronic control mechanism is now in progress, creating a demand for magnetic materials improved in performance and reduced in size as the base of the central component of the control mechanism. Particularly, research on materials having high magnetic property in the middle and high frequency range to allow control at a more precise level and through smaller power is in progress.
In order to have high magnetic properties in the middle and high frequency range, a material must have a high saturated magnetic flux density, high magnetic permeability, and a high electrical resistivity. Metal magnetic materials that have a high saturated magnetic flux density and permeability generally have a low electrical resistivity (10−6 to 10−4 Ω cm). Therefore, the overcurrent loss in the middle and high frequency range is great. Thus, the magnetic property is deteriorated, offering difficulty in the usage as a single element.
Metal oxide magnetic materials are known to have an electrical resistivity (1-108 Ω cm) higher than that of metal magnetic materials, exhibiting smaller over current loss in the middle and high frequency range. Deterioration in the magnetic property is small. However, application is restricted since the saturated magnetic flux density is ⅓ to ½ the saturated magnetic flux density of the metal magnetic material.
In view of the foregoing, there are proposed compound magnetic materials that have a high saturated magnetic flux density, high magnetic permeability and high electrical resistivity by compounding a metal magnetic material with a metal oxide magnetic material to compensate for respective disadvantages.
For example, Japanese Patent National Publication No. 10-503807 discloses a method of forming a compound magnetic material by binding a plurality of compound magnetic particles having a coat of phosphoric acid iron applied to the surface of iron powder with an organic resin such as polyphenylene ether or polyetherimide and amide type oligomer.
In the case where compound magnetic material is employed for the engine control mechanism in an automobile, not only the aforementioned magnetic properties, but also heat resistance is required in view of the high temperature of the engine. However, the compound magnetic material in the above-described publication has the organic resin softened under high temperature since the compound magnetic particles are bound with an organic resin of low heat resistance such as polyphelyne ether or polyetherimide and amide type oligomer. As a result, the bind strength between adjacent compound magnetic particles becomes smaller to result in reduction of the strength of the compound magnetic material.
In view of the above-described problems, an object of the present invention is to provide a compound magnetic material of high heat resistance.